This invention relates to the manufacture of liquid resinous furan-formaldehyde condensation products. The liquid resinous condensation products are useful in manufacturing composite articles such as, for example, foundry sand shapes such as cores and molds. They are also useful in the manufacture of resinforced composite articles such as, for example, fiber glass-bound composite articles.
U.S. Pat. No. 2,306,923 issued to Werner Zerweck, et al. on Dec. 29, 1942 is entitled "Hard Infusible Resinous Condensation Products". The Zerweck patent discloses the acidic condensation of furan and formaldehyde under conditions which lead to hard and infusible resinous condensation products. The method set forth by Zerweck is regarded as being most undesirable, if not unsuitable, for the commercial manufacture of liquid condensation products having viscosities falling within predetermined desired ranges.
Although the Zerweck patent indicates that the condensation of Zerweck may be carried out in several steps and that soluble alkylol compounds may be isolated, and at a subsequent time further advanced, our study led to conclude that under the reactant ratio conditions of Zerwecke the extent of the exothermic polymerization is extremely difficult to control at reasonably elevated temperatures, and that relatively small inadvertent variations in the control of processing parameters, particularly contacting temperatures, result in unacceptable, on occasion, disastrous exothermic advancement of the viscosity of the resin.
W. H. Brown and H. Swatsky authored and article "The Condensation of Furan and Sylvan With Some Carbonyl Compounds" which was published Sept. 1956 in the Canadian Journal of Chemistry (pages 1147-1153). They reported that they obtained difurylmethane from the reaction of furan and formaldehyde. Our study of the Brown and Swatsky reactant ratio condition revealed that the extent of the exothermic polymerization, under the reactant ratio conditions of Brown and Swatsky, is also extremely difficult to control and that relatively small inadvertent variations in the control of processing parameters, particularly contacting temperature, results in unacceptable, on occasion disastrous exothermic advancement of the viscosity of the product.
For example, in a number of tests, closed glass laboratory pressure reaction vessels, though immersed in a heat exchange bath, exploded. Moreover, when relatively small variations in operating parameters result in the uncontrolled advancement of the resins to viscosities in excess of 100,000 cps, with or without sudden high pressures, we regard the process as totally unacceptable for commercial use with respect to the production of liquid resins having viscosities in desired predetermined useful viscosity ranges.
An object of the present invention is to provide useful liquid resins and to provide a method for commercially producing liquid condensation products which are useful in the manufacture of binders in producing commercial composite articles such as foundry cores, molds, reinforced articles, and the like.
A further object of the present invention is to provide a method of condensing furan and formaldehyde which is relatively easily controllable with respect to obtaining desired viscosities of the resulting resinous materials and which method utilizes conditions which are sufficiently reactive to permit substantial and relatively high conversions of the furan in reasonable reaction times. Another object of the present invention is to provide a method of condensing furan and formaldehyde which has not resulted in undesirable run-away polymerization which can result in the solidification of the reaction mass in the reactor. It is a further object of a preferred embodiment of the present invention to provide methods, in accordance with the present invention, by which desired reactivities, as well as desired viscosities can be produced. It is a further object of preferred embodiments of the present invention to provide methods by which condensation products having a desired degree of reactivity ranging from substantially non-reactive with respect to acid catalyzed polymerization to very reactive with respect to acid catalyzed polymerizaton. Also, it is an object of the present invention to provide a method by which it is possible to produce resins having relatively low free formaldehyde levels. It is an additional object of this invention to provide a method to produce resins which are useful as a binder in the manufacture of composite articles such as glass fiber-reinforced articles, and foundry sand shapes, for example.
In accordance with broad aspects of the present invention, formaldehyde is coupled with a furan of the formula: ##STR1## where R and R' are hydrogen, halogen, alkyl, phenyl, alkyl-substituted phenyl, halogen substituted phenyl, hydroxyalkyl, carboxycarboxyalkyl, in which the alkyl constituents have from 1 to 10 carbons, said coupling taking place in the presence of a catalytically effective amount of an acid catalyst and under sufficiently high temperature and for a period of time sufficient to produce a liquid resin under conditions in which the molar ratio of furan/formaldehyde is maintained to provide a substantial molar excess of furan at all times. That is, the conditions are maintained to provide 1.1 or more mole of furan per mole of formaldehyde, preferably maintaining 1.2 or more moles of furan per mole of formaldehyde.
In accordance with the present invention the furans which are coupled with formaldehyde include any furans with both alpha carbons unsubstituted. Particularly preferred, of course, is furan, itself. Also contemplated for use in accordance with the invention are the beta carbon substituted furans such as, for example, 3-chlorofuran, 3-bromofuran, 3-methylfuran, 3-ethylfuran, 3-n-propylfuran, 3-phenylfuran, 3-isopropylfuran, 3-carboxyfuran, 3-p-chlorophenylfuran, 3-p-methylphenylfuran, 3-p-nonylphenylfuran, and di substituted furans, such as, for example, 3-methyl-4-n-butylfuran, 3,-4-di-n-propylfuran, 3,4-di-methylfuran, 3,4-di-ethylfuran, 3,4-di-n-propylfuran, 3,4-di-isopropylfuran, 3-decylfuran, 3-hexylfuran, 3-hydroxymethylfuran, 3-(2-hydroxymethyl)furan, 3,4-di-carboxyfuran 3-(2-propionic acid)furan, and the like.
When the term halogen is employed herein, all of the halogens are contemplated, although of course chlorine is a particularly preferred halogen.
In accordance with the preferred embodiment of the present invention furan and formaldehyde are coupled in he presence of a catalytically effective amount of an acid catalyst and under sufficiently high temperature and for a period of time sufficient to produce a liquid resinous condensation product under conditions in which the molar ratio of furan/formaldehyde is maintained to provide a substantial molar excess of furan at all times. That is, the conditions are maintained to provide 1.1 mole, or more, of furan per mole of formaldehyde. Under more preferable conditions the relative quantities of furan and formaldehyde are such, during the condensation contacting that 1.2 or more moles of furan per mole of formaldehyde is maintained. It is to be understood that, depending on the desired character of the resin, and its intended use, higher, or lower ratios may be preferred in a particular instance.
The contacting can take place in aqueous or nonaqueous systems and furthermore can take place in the presence of inert solvents, if desired. In accordance with preferred aspects of the present invention, the conversion of the furanaldehyde reaction mass is continued until more than 75 percent conversion of the formaldehyde is achieved. In view of the fact that a wide range of relatively high contacting temperatures can be employed, the quantity of catalyst which constitutes a catalytically effective quantity of catalyst also enjoys wide latitude, in accordance with the present invention, and the quantity of catalyst which produces the desired viscosity range under specific time and temperature conditions can be determined easily using simple tests. The wide range of catalyst concentrations which can be employed in accordance with the present invention is further illustrated with the aid of the specific examples herein. For example, from 0.1 to 2.5 grams oxalic acid per 3 moles of furan gave results which were entirely satisfactory in accordance with the present invention.
The acidic materials which can be used in accordance with the present invention to provide the acid catalyst can range from soluble homogeneous catalyst systems to insoluble and even solid heterogeneous catalyst systems. For example, solid acidic materials such as, for example, acidic ion exchange resins are eminently satisfactory for use in accordance with the present invention. In addition, soluble organic and inorganic acids are also imminently satisfactory for use as catalysts in accordance with the present invention. Generally speaking, acids having a pKa member of less than 3 are preferred, while acids having a pKa member of less than 1.7 are most preferred. Acids having higher pKs numbers are also useful, however. Carboxylic acid catalysts such as, for example, benzoic, malonic and oxalic acid, and the like, are preferred acids for use as catalysts. In addition, strong or weak inorganic acid catalysts, such as, for example, phosphoric acid, sulfuric acid, hydrochloric acid, can be employed in accordance with the present invention. Also, inorganic-organic acids such as, for example, p-toluene sulfonic acid can be employed. Also acid salts such as sodium hydrogen sulfate, and Lewis acid such as FeCl.sub.3, can be employed.
Generally speaking, it is preferred that the contacting temperature be at least 30.degree. C. in order to provide a reasonably fast rate of reaction. Temperature below 30.degree. C., and temperatures in excess of 110.degree. C., for example up to 150.degree. C. or higher, are useful but are generally unnecessary in view of the reasonably fast rate of reaction in the 30.degree.-110.degree. C. range.
The contacting temperature is not critical, and the nature of the acid catalyst employed is not critical in accordance with the present invention. In those embodiments of the present invention in which it is desired to reproduce production of resins having viscosities falling within predetermined ranges, the contacting times, contacting temperatures, and the strength of the acid catalysts are somewhat interrelated. The temperature, and time, which is useful in producing a resin having a predetermined viscosity is readily determined, with the aid of the disclosure herein, using simple tests. For example, generally speaking, the weaker acids, as determined by pKa, will require higher contacting temperatures or longer contacting times to produce a resin of the same viscosity as that resin produced by a stronger acid catalyst during contacting at lower temperatures and/or shorter contacting time. Likewise, a lower concentration of a stonger acid can be used at higher temperature to provide a resin of similar viscosity. Generally speaking, within the first few hours of reaction, when comparing a number of tests, as the contacting temperature employed increases, the extent of conversion increases, and the viscosity increases. However, when tests which are carried out to the same degree of conversion are compared, the higher temperatures gave lower viscosity resins. It will be appreciated by those skilled in the art, that given this disclosure, a wide range of conditions including specific range of molar ratios of furan to formaldehyde, temperatures, contacting times can be employed without departing from the spirit or scope of the present invention.
In addition, it is noted that furan is a relatively low boiling liquid and it is highly desirable in accordance with preferred aspects of the present invention to conduct the acidic contacting condensation step in a closed pressurized system wherein the temperature of the furan is maintained at a temperature above its boiling point at atmospheric pressure.
A number of the useful liquid resins produced in accordance with the method of this invention were distilled under greatly reduced pressure, and it was found that most of the resin constitutes substantially non-distillable material which we have not yet characterized.
In the following example all parts are expressed in parts by weight, all temperatures are expressed in degree centigrade and all percents are expressed in percent by weight based on 100 parts by weight of the material then being referred to.